The present invention relates to multiaxis robot controls and more particularly to digital robot controls in which end effector position is controlled through operation of a torque control loop.
In controlling the motion of a robot arm, a command path is typically implemented through the operation of cascaded position, velocity and torque control loops. Output signals from the torque control loop for each joint operate as motor drive commands which cause the joint motor to develop the torque needed to satisfy position, velocity and acceleration commands and move the joint over the commanded joint path. In aggregate, all of the joints move in coordination to satisfy the end effector command path.
During changes in velocity, i.e., during acceleration from rest to slew velocity and deceleration from slew velocity to rest and during changes in slew velocity, a torque command signal must be generated in the control looping to produce motor drive commands. If the torque command is derived only from position and velocity error, actual position error can become relatively large especially during larger changes in velocity such as during startup and stopping. The robot is thus caused to operate with undesired path tracking inaccuracy. Settling error can also be excessive where overshoot occurs.
In the prior art, position inaccuracy due to lag in torque development has been reduced to some extent by employing in each joint control a feedforward torque signal derived from the command acceleration and an inertia constant for the associated link. This type of feedforward control has been employed for example in the digital robot control system exemplary of the type disclosed in U.S. Pat. Nos. 4,763,055 and 4,786,847, which are assigned to the same assignee as the presently assigned invention.
Path control accuracy has nonetheless been deficient with feedforward acceleration adjustment, largely because the prior art tuning constant, i.e. inertia, is a fixed parameter determined by measurements on the particular robot operated by the control. In actuality, link inertia varies with load and with arm configuration. Typically, feedback gains and acceleration commands in the prior art have had to be compromised for worst case arm configurations. Thus, the prior art has achieved some but not enough improvement in path accuracy, and the reason for the shortcoming is that the prior art has not had adaptive capability for addressing variability in the causes of path error.
The present invention advances the state of the art by achieving improved robot path control accuracy with adaptive and preferably digitally implemented feedforward torque control.